1. Field of the Invention
The present invention relates to image processing for chroma-correcting an image.
2. Description of the Related Art
As a scheme for automatically adjusting a chroma level, a technique for changing a chroma gain designated by the user in accordance with the chroma level of an input image is available. This technique detects the chroma level of the input image for one frame, and executes processing for decreasing the user-designated chroma gain if the chroma level is high (for example, Japanese Patent No. 2698493).
However, since the technique of Japanese Patent No. 2698493, detects only a chroma component of an input signal but does not detect any luminance component, it may cause color saturation due to chroma emphasis. FIG. 1 is a graph for explaining this color saturation problem; the abscissa plots the chroma level of an input signal, and the ordinate plots the luminance level of the input signal.
A triangular area shown in FIG. 1 represents an expressible range (to be referred to as a “gamut” hereinafter) of the input signal. Area 1 corresponds to a high-chroma area. Area 2 indicates a chroma area which includes nearly middle chroma levels (to be referred to as “middle chroma levels” hereinafter) and is located near the gamut boundary on the high-luminance side. Area 3 indicates a chroma area which includes middle chroma levels and is located near the gamut boundary on the low-luminance side.
The technique of Japanese Patent No. 2698493, lowers the chroma gain if an input image has a high chroma level. Therefore, the chroma gain of a high-chroma input image whose chroma level belongs to area 1 is lowered, thus preventing or eliminating color saturation. However, the technique of Japanese Patent No. 2698493, does not lower the chroma gain for colors which belong to areas 2 and 3 which do not include any high chroma levels. For this reason, colors which belong to areas 2 and 3 are emphasized by the user-designated chroma gain, and shift in directions indicated by the arrows in FIG. 1. Then, the signal values of colors which deviate from the gamut of an input signal as a result of chroma emphasis are limited to the gamut boundaries, thus causing color saturation.
Also, there is a technique for compressing the values of an image signal to a range between expressible maximum and minimum values when the chroma adjustment result overshoots or undershoots the expressible range (for example, Japanese Patent Laid-Open No. 2001-346222).
However, according to the technique of Japanese Patent Laid-Open No. 2001-346222, colors near the gamut boundaries, in other words, colors in areas 1, 2, and 3 are not so chroma-emphasized, and colors other than those colors undergo strong chroma emphasis. Since this chroma emphasis processing is applied for each pixel, the chroma levels of the entire image shift to the vicinity of the gamut boundaries, and details of colors near the gamut boundaries may be lost.
Since the techniques of both Japanese Patent No. 2698493, and Japanese Patent Laid-Open No. 2001-346222, control chroma emphasis regardless of the gamut of a display apparatus, they suppress the chroma emphasis strength of even colors displayable due to a broad gamut of the display apparatus, and cannot obtain a chroma correction result desirable to the user.